1. Field of the Invention
The present invention relates to the operation of a proton exchange membrane fuel cell, and in particular, a system and method for using fiber optic fluorescence spectroscopy to detect changes in the Nafion® membrane water content in a fuel cell at several operating conditions.
2. Brief Description of the Related Art
Polymer Electrolyte Fuel Cells (“PEFCs,” which are also referred to as Proton Exchange Membrane Fuel Cells, or “PEMFCs”) are well suited for a variety of applications by virtue of their efficiency, environment friendly nature and high power densities.
A PEFC generally consists of a membrane sandwiched between two electrodes that are typically made from platinum or platinum-ruthenium supported on carbon, along with ionomeric material or electrolytes to promote protonic conductivity, such as sulfonic acid groups disposed in the membrane. Porous carbon backings (with a micro porous carbon layer on one side) are used to distribute the fuel and oxidant gases uniformly throughout the active electrode area, to conduct electrons and facilitate water transport, among other things. The gases enter the system through flow fields machined into graphite plates, which also serve as current collectors.
Fuel, typically hydrogen (H2) or methanol, is electrochemically oxidized at one of the electrodes, which is called the anode, to form protons and electrons. The protons diffuse through the polymer electrolyte membrane (“PEM”) to the other electrode, called the cathode, where they combine electrochemically with the oxidant, which is usually air, to produce water. The electrons needed to complete this reduction reaction are directed to the cathode via an external circuit electrically connecting the anode and cathode.
One of the most common PEMs in use today for solid electrolyte for H2/O2 polymer electrolyte fuel cells is Nafion®, a commercially available perfluorinated sulfonic acid ionomer manufactured by E.I. du Pont de Nemours & Co. Nafion® is a copolymer of tetrafluoroethyelene and sulfonyl fluoride vinyl ether and has reverse micelle morphology in the dry state, where the ionic clusters are dispersed in a continuous tetrafluoroethyelene phase. The Teflon-like inert hydrophobic backbone provides chemical, mechanical and thermal stability, whereas the pendant sulfonic acid group of the vinyl ether imparts hydrophilicity and, most importantly, proton conductivity.
While this membrane performs very well in a saturated environment, its charge carriers are hydrated protons, resulting in membrane proton conductivity that is largely influenced by water content. Proton conductivity decreases considerably when the water content in the operating environment is low (i.e., low relative humidity conditions). When the water content in the membrane is at or above a critical level, the ionic domains swell with water absorption to form interconnected proton-conducting channels. The conductivity increases because this swelling reduces the separation between the micelles and aids in the transport of proton. However, if the water content is too great the ionic groups become diluted which decreases the concentration of the protons therein and reduces proton conductivity.
The proton transport in the membrane is known to occur via hydronium ions (electro-osmotic drag), which results in drying of the anode. To prevent anode drying, the hydrogen gas is typically saturated with water vapor. The water generated at the cathode and that carried by electro-osmotic drag create a water gradient resulting in back diffusion of the water towards the anode. The water distribution throughout the membrane is determined by the membrane water uptake, the spatial variations in fuel (i.e., H2) supply and the interplay between electro-osmotic drag and back diffusion. Uneven distribution of the feed in the gas distribution channels and catalyst poisoning can lead to non-uniform electrochemical reaction and uneven water content in the x-y plane of the membrane. In sum, water management is crucial, and determining the in-situ water content can be used to greatly improve the efficiency of fuel cell operation.
Gravimetric analysis, Near-IR and NMR spectroscopy can quantify the water content, but in-situ measurements are difficult. An estimate of the water content inside the cell can be obtained using water mass balancing, that is, by measuring the relative humidity of the incoming and outgoing streams at steady state, but this does not truly represent the membrane water content as water can condense in the gas distribution channel and gas diffusion layer. The membranes within the fuel cell are thin and coated with electrodes on either side, and the cell is clamped under high pressure for a good electrical circuit and made leak proof, making incorporation of a conventional sensor within the fuel cell therein extremely difficult.
Thus, what is needed is a system and method for accurately measuring water content in PEFCs without compromising the integrity or disturbing the normal fuel cell arrangement. As is readily apparent, such a system and method would provide higher efficiency PEFC operation and yield advantages in fuel cell applications which may not have been contemplated due to the limitations described above.